Media mail has a mission.

I was at the post office this morning mailing a package to a friend. The postal worker had a lot of questions for me: Anything liquid, fragile, potentially hazardous? Are you sending this priority or first class? Do you need insurance? No, neither, no. Since I was mailing a book, I knew that the package was eligible for "book rate" aka media mail, which meant that it would be less expensive to ship. And since there was no urgency for the book to arrive fast, book rate was perfect. As I swiped my credit card, forking over about $3 instead of $5, I realized that I didn't know why things like books can be sent cheaper via USPS. So, I asked my postal friend, Kim. Kim told me that media mail exis